[time-nuts] Raspberry Pi tweaks and custom kernel, was RE: PPS for NTP Server - How Close Is "Good Enough"?

Bob Camp kb8tq at n1k.org
Sun Jun 14 13:45:46 UTC 2015


Just to be clear, your peer delay numbers for the Pi show and improvement from 0.5 ms down to 0.3 ms with some tweaks. A “non-usb” ethernet setup can get those delays down much further. In the context of the original question “tweaking the USB drivers” this is what was
being addressed.


> On Jun 14, 2015, at 2:17 AM, David J Taylor <david-taylor at blueyonder.co.uk> wrote:
> Hi
> At best, simply because of the way that USB interface works, you can go from the ~1/2 ms range into the ~1/4 ms range. Rewriting the low level drivers may be required.  This compares to an equivalent lag on a device with a built in ethernet of < 0.001 ms. The easy thing to do is to simply kill off everything else that is running on the device. That goes double for anything running on USB (keyboards / mice etc).
> Since you can get boards with built in (integrated in the MCU)  ethernet for < $50, it’s not clear how much tweaking time this is worth. You don’t get any real use out of the Pi’s fancy graphics in this case. The board really is not optimized for doing this sort of thing. Also - check the threads on Pi heat sinking and boards burning out. If you plan to use this full time - invest the money in cooling now.
> Bob
> ==================================
> Bob,
> Thanks for your comments.  I made a simple comparison between the RPi and the BBB with help from the folks here, and wrote it up here:
> http://www.satsignal.eu/ntp/BBB-vs-RPi.html
> Using NTP, the reported RMS offset on a client on the same LAN was 18 microseconds (BBB) and 39 microseconds (RPi), and the mean RMS jitter was 7 microseconds (BBB) and 29 microseconds (RPi).  So the USB/Ethernet does make the RPi less good as a server, but depending on your application it may still be more than adequate.  Neither system provides "microsecond" level, both are more than adequate for "tenth millisecond" level.
> The BBB generated sufficient RF interference to stop a near-on-board GPS from working.
> I have had no overheating troubles with the 9 RPi systems here, none have needed added cooling, and none have burnt out.  One is even running Windows-10 IoT.
> Cheers,
> David
> -- 
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk 
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